1. Field of the Invention
This invention relates to well working fluids useful in drilling, completing, or workover of oil and gas wells, and specifically, to water-base fluids containing one or more water soluble polyalkylene glycols, and one or more polysaccharide viscosifiers therein.
2. Prior Art
During the drilling of an oil and gas well, including drilling into a hydrocarbon-bearing formation and during conducting completion and workover operations in a well, it is essential to utilize a fluid in the well bore (i.e., borehole) to minimize the effect of the fluid on the formations contacted by the fluid. See for example the paper by Eric van Oort entitled "Physico-Chemical Stabilization of Shales", Society of Petroleum Engineers (SPE) paper no. 37263.
Water based fluids containing various glycols and/or glycol ethers have been proposed, and are being used, which have much better shale stabilizing characteristics than prior known water base fluids. See for example the following references: Perricone et al., U.S. Pat. No. 4,963,273; Enright et al., U.S. Pat. No. 5,007,489; Melear et al., U.S. Pat. No. 5,120,708; "Water-Based Glycol Systems Acceptable Substitute for Oil-Based Muds", R. Bland, Oil & Gas Journal, Jun. 29, 1992, pp. 54-56, 58, 59; "TAME: A New Concept in Water-Based Drilling Fluids for Shales", J. D. Downs et al., SPE 26699; "Glycol-Enhanced Water-Based Muds: Case History To Demonstrate Improved Drilling Efficiency in Tectonically Stressed Shales," A. J. Twynam et al., IADC/SPE 27451; "Mechanism of Shale Inhibition by Polyols in Water Based Drilling Fluid", P. I. Reid et al., SPE 28960; "Low Salinity Polyglycol Water-Based Drilling Fluids as Alternatives to Oil-Based Muds", R. G. Bland et al., SPE/IADC 29378; "Low Salinity Polyglycol Water-Based Drilling Fluids as Alternatives to Oil-Based Muds", R. Bland et al., IADC/SPE 26400; "Glycols Applied In A Broad Range of Drilling Fluids," S. Seaton, Hart's Petroleum Engineer International, March, 1997, pp. 57-59, 61; "Improving HTHP Stability of Water Based Drilling Fluids", Eric van Oort et al., SPE/IADC 37605.
It is well known that certain biopolymer-containing fluids are shear thinning, exhibiting a high low shear rate viscosity and a low high shear rate viscosity. A near zero shear rate (0.06 to 0.11 sec.sup.-1) viscosity provides a numerical value related to the ability of a fluid to suspend particles or cuttings under static conditions. Conversely, viscosity measured at shear rates above 20 sec.sup.-1 relates to the hole cleaning capacity of a fluid under annular flow conditions. Such fluids have been eminently successful for use in high angle and horizontal drilling. See for example: (1) "Drill-In Fluids Improve High-Angle Well Production", Supplement to Petroleum Engineer International, March, 1995, p. 5-11; and (2) "Soluble Bridging Particle Drilling System Generates Successful Completions in Unconsolidated Sand Reservoirs", J. Dobson and D. Kayga, presented at the 5.sup.th International Conference on Horizontal Well Technology, Amsterdam, The Netherlands, Jul. 14-16, 1993.
It is disclosed in Dobson, Jr. et al. U.S. Pat. No. 5,616,541 to utilize an amorphous silica as a viscosifier in calcium- and zinc-containing high density brine fluids. It is disclosed in co-pending United States Patent application of James W. Dobson, Jr. et al. Ser. No. 08/512,675 filed Aug. 25, 1995, to provide calcium- and zinc-containing brine fluids containing one or more polysaccharide polymer viscosifiers and an amorphous silica viscosifier therein.
Dobson Jr. U.S. Pat. No. 5,514,644 discusses the thermal instability of aqueous polysaccharide-containing fluids.